The overall objective of the proposed research is to isolate and characterize marrow dependent (M) cells which mediate marrow allograft reactivity in mice. Cell separation techniques will be utilized to deplete the spleen or bone marrow of T cells, B cells and macrophages. The residual population, which is enriched in natural killer (NK) cells against tumor targets, will be tested in a variety of in vitro assays which seem to reflect M cell activity. With the isolated M cells an attempt will be made to prepare a specific anti-M cell serum. In vitro assay systems which apparently reflect M cell-like activity (NK cells, mitogen induced cellular cytotoxicity) will be compared and analyzed to evaluate if they are indeed mediated by M cell populations. Such an analysis will involve testing cells from mice injected with 89 SR which selectively suppresses M cell functions in vivo. A new assay will be developed which hopefully will truly reflect the biological properties and functions of M cells in vitro. This assay is based on the premise that spleens of irradiated mice which are actively rejecting an incompatible marrow graft, would be an enriched source of M cells. Spleen cells from such mice will be used as killer cells with appropriate target cells to investigate M cell mediated killing in vitro.